The research proposed here deals with the effects of lesions on the forebrain visual system of reptiles and birds on visual discriminative behavior. These experiments are directed toward understanding how visual function has evolved in amniotes. Recent experiments from this laboratory have demonstrated that forebrain lesions of the tectofugal pathway in turtles produce deficits in pattern and visual intensity discrimination. These deficits appear to be more severe after thalamic than after telencephalic lesions. The proposed experiments in turtles will examine the effect of these lesions on acuity difference thresholds, auditory discrimination, and the ability to learn reversals of a visual discrimination and to shift dimensions. The effects of lesions of the dorsal cortex which produce no deficits on simple intensity and pattern discrimination, will also be studied on these tasks. An experiment on pigeons is also proposed. This experiment is designed to investigate the stimulus generalization gradients obtained from pigeons with telencephalic lesions of the thalamofugal visual systems. The data from this experiment will be compared with those obtained in mammals on a similar task.